Select desired size, add it to the cart and when directed to the payment page you can enter the quantity for that item.
If you want to add more items to the cart, select "continue shopping" located in the upper right hand corner of the cart.
Repeat for each item and size needed.
You can review the contents of your cart by using the "View Cart" button. To view size information on the items in your cart, click the "more" button under the item description.
When all items are in the cart, you can make payment with your Credit Card or PayPal account.